Iron/Zinc-Co-catalyzed Directed Arylation and Alkenylation of C(sp3)-H Bonds with Organoborates

Article abstract of DOI:10.1021/acscatal.6b02927

An iron(III) salt, (Z)-1,2-bis(diphenylphosphino)ethene or its electron-rich congener, (Z)-1,2-bis[bis(4-methoxyphenyl)phosphine]ethene, and a zinc(II) salt catalyze the arylation, heteroarylation, and alkenylation of propionamides possessing an 8-quinolylamide group with organoborate reagents in the presence of 1,2-dichlorobutane as oxidant at 70 °C. Stoichiometric experiments provided evidence for the involvement of an organoiron(III) species as a key intermediate for C-H activation and C-C bond formation.

Full text of DOI:10.1021/acscatal.6b02927

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Letter
Iron/Zinc-Cocatalyzed Directed Arylation and
Alkenylation of C(sp3)–H Bonds with Organoborates
Laurean Ilies, Yuki Itabashi, Rui Shang, and Eiichi Nakamura
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02927 • Publication Date (Web): 22 Nov 2016
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Iron/Zinc-Cocatalyzed Directed Arylation and Alkenylation of
C(sp3)–H Bonds with Organoborates
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Laurean Ilies*, Yuki Itabashi, Rui Shang, and Eiichi Nakamura*
Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033
ABSTRACT: An iron(III) salt, (Z)-1,2-bis(diphenylphosphino)ethene or its electron-rich congener, (Z)-1,2-bis[bis(4-
methoxyphenyl)phosphine]ethene, and a zinc(II) salt catalyze the arylation, heteroarylation, and alkenylation of propionamides
possessing an 8-quinolylamide group with organoborate reagents in the presence of 1,2-dichlorobutane as oxidant at 70 °C. Stoi-
chiometric experiments provided evidence for the involvement of an organoiron(III) species as a key intermediate for C–H activa-
tion and C–C bond formation.
KEYWORDS: iron catalysis, zinc catalysis, C(sp3)–H activation, boron compound, alkenylation, amide
Among the rapidly expanding repertoire of metal-catalyzed
C–H activation reactions,1 in particular C(sp2)–H activation,2
examples of C(sp3)–H activation are still scarce and their
scope is rather narrow.3 For example, a standard combination
of palladium and organoboron reagents4 is very effective for
introducing an aryl group,5 but is applicable to the introduction
of only a limited variety of alkenyl groups.6 An iron-catalyzed
C(sp3)–H activation protocol that we developed recently by
the use of organozinc7 or -aluminum reagents8 is not applica-
ble for the alkenylation reaction. We report here that a combi-
nation of an organoboron reagent, an iron catalyst,9 a diphos-
phine ligand, and a zinc cocatalyst effects arylation, heteroary-
mild oxidant, to produce the corresponding β-phenylated am-
ide (2) in 90% yield after isolation by column chromatography
(eq. 1). A trace amount (5%) of starting material was recov-
ered. Homocoupling of the organometallic reagent, which
accounted for a major material loss when an organozinc rea-
gent was used,7 was observed in 2%, suggesting that iron(III)
species is not reduced by the borate reagent (see below). No-
tably, under similar reaction conditions, an alkenylboron rea-
gent also reacted well with 1 to give β-alkenylated 3 in 80%
yield (eq. 2), while this reaction required 20 mol % of catalyst
and a more electron-rich cis-ethylenediphosphine (MeO-
dppen) designed for this study, to achieve 80% yield (with
dppen, 50% yield).
lation, and alkenylation at the
β-C–H bond in α,α-
disubstituted propionamides. A new ligand, electron-rich (Z)-
1,2-bis[bis(4-methoxyphenyl)phosphine]ethene (MeO-dppen)
improved the catalytic efficiency of the alkenylation reaction.
We have previously reported that an organozinc reagent
functions as an aryl donor in the iron-catalyzed C–H activation
of propionamides,7 but the reaction was restricted to arylation,
accompanied by formation of biaryl homocoupling products.10
We hypothesized that this problem arises from the use of a
highly reactive diarylzinc reagent, which delivers two organic
groups to iron (cf. D, X = Ph in Figure 1), causing reductive
elimination to a low-valent iron species11 (such as E in Figure
1) and biaryl. We envisioned that a milder organoborate rea-
gent may deliver only one organic group to a high-valent iron
center in each step of a catalytic cycle, which will suppress the
diaryl formation and hence formation of catalytically inactive
reduced iron species. The role of organoborates for this pur-
pose was hinted by their successful use for C(sp2)–H activa-
tion, recently reported by us.12
The key reaction parameters are described in Table 1 and
Supporting Information. As seen from the overall reactivity
profile, Zn(II) (>0.2 equiv) was found to be essential, probably
because it facilitates deprotonation of the amide’s nitrogen and
After considerable experimentation, we found that a propio-
namide bearing a bidentate 8-quinolylamide directing group
(Q)13 (1) reacts at 70 °C with a borate reagent prepared in situ
from Ph-Bpin (pin = pinacolato) and BuLi14 in the presence of
a catalytic amount of Fe(acac)3/bidentate diphosphine ligand
transfer of the phenyl group from borate to iron.12,15 16 The use
,
of BuMgBr instead of BuLi/Zn(II) shut off the reaction (entry
1). We note that the aryl or alkenyl group was transferred ex-
clusively to the substrate, while the butyl group was not. Other
((Z)-1,2-bis(diphenylphosphino)ethene: dppen),
a catalytic
amount of Zn(OAc)2, and 1,2-dichloroisobutane (DCIB) as a
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using tridecane as an internal standard. cThis borate (6 equiv) was
used instead of Ph-Bpin/base.
borates such as phenyltrifluoroborate (entry 2) or phenylcy-
clic-triolborate (entry 3) were ineffective. Zinc chlo-
ride/TMEDA complex and zinc acetate gave similar results
(entries 5 and 6), and we chose the latter in the present study.
A catalytic amount of zinc salt sufficed to promote the reac-
tion (entries 6 and 7). The overall data showed that dppen (en-
tries 7–11) and the more electron-rich MeO-dppen (eq. 2 and
entry 8) are suitable ligands for this reaction. A diphosphine
1
2
3
4
5
6
Table 2. The Effect of Solvent Amount on Phenylation of
Amide 1 with 10 mol % of Catalysta
7
8
bearing
a
benzene
backbone
(dppbz
=
1,2-
bis(diphenylphosphino)benzene, entry 9) and a bipyridine
derivative (dtbpy = 4,4′-di-tert-butyl-2,2′-bipyridyl, entry 12)
also functioned as a ligand, but with lower efficiency. A
monodentate phosphine (entry 10) was entirely inefficient and
the reaction did not proceed at all in the absence of a ligand.
Notably, a diphosphine bearing a saturated backbone, dppe
(1,2-bis(diphenylphosphino)ethane) was entirely inefficient
(entry 11), suggesting stabilization of a low-valent organoiron
species through spin delocalization over the ligand,17 as previ-
ously reported.12 The reaction using a catalytic amount (20
mol %) of Fe(acac)2 or FeCl2 did not proceed at all. Four equiv
of boron reagent was necessary, and the reaction using three
equiv gave the product in 1%. One equiv deprotonates the
amide’s nitrogen, one equiv acts as a base to assist C–H cleav-
age, and one equiv is used for arylation. 1,2-Dichlorobutane
was optimal as the oxidant, as previously observed.18
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aReaction conditions: 1 (0.20 mmol), Ph-Bpin (6 equiv), BuLi
(6 equiv), Fe(acac)3/dppen (10 mol %), DCIB (2 equiv) in THF at
the specified concentration, 70 °C, 21 h. bThe yield was estimated
by GC using tridecane as an internal standard.
As shown in Table 3, α,α-disubstituted pivalamides reacted
with aryl, heteroaryl, and alkenyl boronates. Both electron-rich
(entries 1–3) and electron-deficient (entries 4–6) aryl groups
were efficiently introduced, and dimethylamine (entry 3) and
halide groups including bromide (entries 5–7) were well toler-
We found a marked effect of the concentration on the prod-
uct yield (Table 2), which was not the case in our previous
study of C–H activation of aromatic amides with the same
borate reagent.12 Dilution decreased the yield, and the optimal
concentration was 0.80 mol/L of 1 in THF; further increasing
the concentration resulted in precipitation.
ated. 1-Methylcyclohexylamide (entry 12) and
phenylamide (entry 13) that was unreactive in the reaction
with organozinc reagents7 were also
-arylated with the phe-
a
2-
β
nyl boronate. The advantage of organoborate reagents is high-
lighted by the successful introduction of heteroaryl and
alkenyl groups, which was not possible with organozinc rea-
gents. For example, a thienyl group (entry 8) was introduced
in high yield. Other heteroarylboronates such as 3-furanyl and
or 4-pyridyl reacted sluggishly. Simple alkenyl reagents, a
difficult substrate for the functionalization of C(sp3)–H
bonds,6 reacted stereospecifically, as illustrated by the intro-
duction of (E)-propenyl (entry 9) and (E)-2-cyclohexylethenyl
groups (entry 11) with high (E) selectivity, and the reaction of
a stereolabile (Z)-propenylboronate (E:Z ratio = 7:93) with
83% retention of stereochemistry (E:Z ratio of the product =
23:77, entry 10).
Table 1. The Effect of Reaction Parameters on Phenylation
of Amide 1 with 20 mol % of Catalysta
aReaction conditions: 1 (0.2 mmol), Ph-Bpin (4.1 equiv), base
(4 equiv), Fe(acac)3/ligand (20 mol %), DCIB (2 equiv) in THF
b
(ca. 0.2 mol/L), 70 °C, 24 h. The yield was estimated by GC
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corresponding zinc amide, together with BuBpin and benzene
Table 3. Iron-Catalyzed Reaction of Pivalamides with Var-
ious Organoboronates using Dppen and MeO-dppena
(cf. Figure 1a). Accordingly, the catalytic reaction of a pre-
formed zinc carboxamide also proceeded (SI). A zinc salt also
assists boron/iron transmetalation.12,15,16 All this evidence
points to the involvement of a high-valent organoiron species
such as Fe(III) as the active species that cleaves the C–H
bond.8,12
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A possible mechanism of the reaction stoichiometric both to
iron(III) and zinc(II) cations is proposed in Figure 1, based on
the above experiments, and a related study on the reaction
stoichiometry.12 Zinc-assisted deprotonation gives zinc amide
C (Figure 1a), and then boron/iron transmetalation gives or-
ganoiron(III) intermediate D. The lack of biphenyl suggests
that D is a monophenyliron complex (X = OAc or acac); by
contrast, when a more nucleophilic base such as Ph2Zn is
used,7 a diphenyliron complex (D, X = Ph) forms, and its re-
ductive elimination gives the low-valent organoiron E and
biphenyl. Conversion of D to the product K via an oxidative
addition mechanism (via F and an iron(V) species G) is im-
probable. Alternatively, C–H activation of complex D through
,
σ-bond metathesis19 20 via H to metallacycle I is more proba-
ble. Ferracycle I accepts another phenyl group from phenyl-
boronate and then undergoes reductive elimination to create
the C–C bond and generate organoiron(I) intermediate J. This
apparently unstable iron(I) species may be stabilized by elec-
tron delocalization over the ligand backbone.12,17,8
aReaction conditions: amide (0.40 mmol), Ph-Bpin (4.1 equiv),
base (4.0 equiv), Fe(acac)3/ligand (10 mol % dppen for arylation,
20 mol % MeO-dppen for alkenylation), DCIB (2 equiv) in THF
(0.5 mL), 70 °C. bYield of the isolated product.
Stoichiometric reactions of amide 1 with phenyl borate (4
equiv) using 1 equiv each of Fe(acac)3 and dppen in the ab-
sence of any oxidant (eq. 3) revealed several mechanistic in-
sights. One equiv of borate is required for deprotonation of the
amide’s nitrogen, 1 equiv acts as a base for C–H cleavage, and
1 equiv is necessary for C–C bond formation. The reaction
requires both 1 equiv of both Fe(acac)3 and Zn(OAc)2. The
stoichiometric reaction in the absence of oxidant gave the C–H
phenylation product in 47% yield and BuBpin, indicating that
the DCIB oxidant used in the catalytic reaction is not essential
for the C–H activation and the C–C-forming process, and that
it is mainly required to reoxidize the iron species. Only a small
amount (10%) of biphenyl formed, suggesting that phenyl
boronate does not reduce iron. Interestingly, the use of a cata-
lytic amount of Zn(II) did not promote the reaction at all. This
may suggest the involvement of zinc as a cocatalyst for the C–
H activation step. We consider that the zinc salt plays a dual
role: as we previously observed through 11B NMR studies,12
deprotonation of the amide’s nitrogen by the borate alone is
slow, but it is greatly accelerated by a zinc salt to give the
Figure 1. A possible reaction mechanism. The coordination
stereochemistry was chosen arbitrarily.
In conclusion, we have developed an iron-catalyzed reaction
of α,α-disubstituted propionamides bearing a bidentate direct-
ing group with aryl, heteroaryl, and alkenyl boron reagents.
The success of this reaction relies on the use of a bidentate
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directing group and a diphosphine ligand bearing a conjugated
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backbone, together with a mild organoboron reagent to pre-
vent reduction of the organoiron species and to maintain the
high-valent state of the organoiron(III) active species. The
catalyst turnover was improved by increasing the electron
density on the phosphine ligand and by increasing the concen-
tration of the substrate. Through the stoichiometric experi-
ments, we provided additional evidence for the involvement of
an organoiron(III) species (perhaps complexed with a zinc(II)
center) as a key intermediate for C–H activation and C–C
bond formation.
(d) Liu, Y.-J.; Zhang, Z.-Z.; Yan, S.-Y.; Liu, Y.-H.; Shi, B.-F.
Chem. Commun. 2015, 51, 7899–7902.
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Soc. 2013, 135, 6030–6032.
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7660–7663.
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ASSOCIATED CONTENT
Supporting Information
Experimental procedures and physical properties of the com-
pounds. The Supporting Information is available free of charge on
the ACS Publications website.
AUTHOR INFORMATION
(
10) (a) Cahiez, G.; Chaboche, C.; Mahuteau-Betzer, F.; Ahr, M. Org.
Lett. 2005, 7, 1943–1946. (b) Nagano, T.; Hayashi, T. Org. Lett.
2005, 7, 491–493. (c) Cahiez, G.; Moyeux, A.; Buendia, J.; Du-
plais, C. J. Am. Chem. Soc. 2007, 129, 13788–13789.
Corresponding Author
*laur@chem.s.u-tokyo.ac.jp; nakamura@chem.s.u-tokyo.ac.jp
Author Contributions
(11) Bedford, R. B. Acc. Chem. Res. 2015, 48, 1485–1493.
(12) Shang, R.; Ilies, L.; Asako, S.; Nakamura, E. J. Am. Chem. Soc.
2014, 136, 14349–14352.
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Soc. 2005, 127, 13154–13155. (b) Daugulis, O.; Roane, J.; Tran,
L. D. Acc. Chem. Res. 2015, 48, 1053–1064. (c) Rouquet, G.;
Chatani, N. Angew. Chem., Int. Ed. 2013, 52, 11726–11743.
(14) Kobayashi, Y.; Mizojiri, R. Tetrahedron Lett. 1996, 37, 8531–
8534.
The manuscript was written through contributions of all authors.
All authors have given approval to the final version of the manu-
script.
Funding Sources
MEXT KAKENHI Grant Number 15H05754
MEXT KAKENHI Grant Number 26708011
JSPS Research Fellowship for Young Scientists No. 26-04342
(15) Zn additives: (a) Bedford, R. B.; Hall, M. A.; Hodges, G. R.;
Huwe, M.; Wilkinson, M. C. Chem. Commun. 2009, 6430–6432.
(b) Bedford, R. B.; Gower, N. J.; Haddow, M. F.; Harvey, J. N.;
Nunn, J.; Okopie, R. A.; Sankey, R. F. Angew. Chem., Int. Ed.
2012, 51, 5435–5438.
(16) Mg additives: (a) Hatakeyama, T.; Hashimoto, T.; Kondo, Y.;
Fujiwara, Y.; Seike, H.; Takaya, H.; Tamada, Y.; Ono, T.; Naka-
mura, M. J. Am. Chem. Soc. 2010, 132, 10674–10676. (b) Hash-
imoto, T.; Hatakeyama, T.; Nakamura, M. J. Org. Chem. 2012,
77, 1168–1173. (c) Hatakeyama, T.; Hashimoto, T.; Kathriarach-
chi, K. K. A. D. S.; Zenmyo, T.; Seike, H.; Nakamura, M. Angew.
Chem., Int. Ed. 2012, 51, 8834–8837. (d) Bedford, R. B.; Brenner,
P. B.; Carter, E.; Carvell, T. W.; Cogswell, P. M.; Gallagher, T.;
Murphy, D. M. M.; Neeve, E. C.; Nunn, J.; Pye, D. R. Chem.—
Eur. J. 2014, 20, 7935–7938.
(17) Blanchard, S.; Derat, E.; Murr, M.; Fensterbank, L.; Malacria,
M.; Mouries-Mansuy, V. Eur. J. Inorg. Chem. 2012, 376–389.
(18) (a) Norinder, J.; Matsumoto, A.; Yoshikai, N.; Nakamura, E. J.
Am. Chem. Soc. 2008, 130, 5858–5859. (b) Yoshikai, N.;
Matsumoto, A.; Norinder, J.; Nakamura, E. Angew. Chem., Int.
Ed. 2009, 48, 2925–2928. (c) Ilies, L.; Asako, S.; Nakamura, E. J.
Am. Chem. Soc. 2011, 133, 7672–7675. (d) Yoshikai, N.; Asako,
S.; Yamakawa, T.; Ilies, L.; Nakamura, E. Chem.—Asian J. 2011,
6, 3059–3065. (e) Shang, R.; Ilies, L.; Matsumoto, A.; Nakamura,
E. J. Am. Chem. Soc. 2013, 135, 6030–6032. (f) Gu, Q.; Al Ma-
mari, H. H.; Graczyk, K.; Diers, E.; Ackermann, L. Angew.
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ACKNOWLEDGMENTS
We thank MEXT for financial support (KAKENHI No.
15H05754 to E.N. and No. 26708011 to L.I.). R.S. thanks the
Japan Society for the Promotion of Science for a Research Fel-
lowship for Young Scientists (26-04342). This work was partially
supported by CREST, JST.
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84x30mm (300 x 300 DPI)
ACS Paragon Plus Environment